1. Field of the Invention
The present invention relates broadly to a gas sensor of the thermal conductivity type for use in the quantitative analysis of a gaseous mixture. More particularly, this invention is concerned with a gas sensor of the class mentioned which may suitably be used for measurement of fuel vapor content of a fuel-air mixture in an automotive emission control system.
2. Description of the Prior Art
Conventional evaporative emission control system of automobiles generally includes a charcoal canister connected to a fuel tank as schematically shown in FIG. 1. When the automotive engine is not running, fuel vapor generated in the fuel tank is forwarded via a fuel cut-off valve and a two-way valve to the canister for adsorption by activated charcoal. As the engine is restarted, a purge control valve in a purge line permits air to be drawn through the canister under the action of an intake vacuum whereby fuel is desorbed and purged from the canister and is delivered to the engine intake system for combustion in the engine cylinders.
In order to reduce exhaust emissions as well as to achieve fuel economy, it is desirable that fuel from a fuel line be metered by a charge forming device, such as an electronically controlled fuel injection system, on account of the amount of fuel coming from the purge line. To this end, it is desirable to detect the amount of fuel flowing through the purge line. It is therefore contemplated to provide a gas sensor in the purge line in order to continuously monitor the fuel vapor content.
Known in the art are various types of gas sensors, including semiconductor type, catalytic combustion type and thermal conductivity type. Among them, gas sensors of the thermal conductivity type are known as being free from deterioration and contamination and being responsive to a wide range of gas composition.
Gas sensors of the thermal conductivity type are well-known known in the art as disclosed, for example, in U.S. Pat. No. 4,813,267 granted to Norem et al, Japanese Patent Kokai Publication Nos. 55-7698 and 57-16343, and Japanese Patent Kokoku Publication No. 5-18055. Gas sensors of the thermal conductivity type are designed to issue a voltage signal indicative of the composition of a two-components gas mixture by making use of the principle that all gases have an intrinsic thermal conductivity. Table 1 given below indicates intrinsic thermal conductivity of exemplary gases.
TABLE 1 ______________________________________ (Thermal conductivity of gases at 300.degree. K. under the standard atmospheric pressure) Thermal Conductivity Gas Species (mW/m .multidot. K) ______________________________________ hydrogen 181 helium 153 methane 33.55 oxygen 26.29 nitrogen 25.98 air 26.14 carbon monoxide 24.87 ammonia 24.6 ethylene 20.5 propane 18.4 carbon dioxide 16.55 isobutane 16.3 ______________________________________
As the thermal conductivity of a two-component gas mixture is proportional to the mixture ratio, the content of one component in the mixture can be derived once the thermal conductivity of the mixture is detected.
More specifically, a gas sensor of the thermal conductivity type typically includes a pair of elements which are generally identical in structure, size and thermal property with one another. Each element comprises an enclosure defining a chamber therein and serving as a heatsink. In each chamber, an electrically-heated hot member, such as a hot wire of platinum or a thermistor, having a temperature responsive electrical resistance is placed. The hot members of both elements are incorporated into a four-resistor Wheatstone bridge circuit and are heated by an electric current of controlled, constant intensity.
In use, the hot member of one element, known as the sensing element, is brought into contact with a gas mixture to be tested, with the hot member of the other element, known as the compensating element, being in contact with a reference gas. As both of the hot members are constantly energized by an electric current of a controlled predetermined intensity, a substantially equal amount of heat will be generated in each of the hot members. Given the velocity of gas to be zero so that heat transfer by way of the forced convection process does not occur, heat emanating from respective hot members will be transferred by way of the thermal conduction of surrounding gas to respective enclosures serving as heatsinks which are subjected to the ambient temperature. In a thermally equilibrated condition, the temperature of the hot member will be dependent on the thermal conductivity of gas surrounding the hot member.
Thus, in the compensating element, the temperature of the hot member will be constant since the heater is in contact with a reference gas which has a given thermal conductivity.
On the other hand, in the sensing element wherein the hot member is in contact with a gas mixture to be tested, any change in the composition of the mixture will result in a change in the thermal conductivity thereof. This brings about a change in the amount of heat transfer per unit time and, hence, a change in the hot member temperature which, in turn, results in a change in the electrical resistance thereof, so that an unbalanced voltage potential is developed across the output terminals of the Wheatstone bridge circuit.
The mixture ratio of a particular gas component in a two-component mixture may then be determined according to the difference between the output voltage developed with the mixture to be tested and the full-scale output voltage which would be developed with a pure gas consisting solely of the particular gas component of interest.
However, the problem associated with a gas sensor of the thermal conductivity type as applied to the measurement of the fuel vapor content in a fuel-air mixture is that the output voltage of the Wheatstone bridge is not always reliable. It has been observed that, when a mixture of a given fuel content is tested with the hot member being heated with varying current intensity, the output potential has fluctuated considerably. This makes the measurement of the fuel vapor content practically impossible.
Another problem which must be overcome in designing a commercially feasible gas sensor for use in an automotive emission control system is related to the pressure dependency of the output signal. Since the intake pressure of the engine varies significantly in response to varying operating conditions of the engine, the gas sensor is subjected to a wide range of pressure variation. Measuring errors which would result from intake pressure variation is non-negligible in properly controlling the fuel-to-air ratio of the combustible mixture.